Recently, optical Differential Phase Shift Keying (DPSK) modulation has received considerable attention by the telecommunications industry primarily due to its increased sensitivity over commonly used On-Off-Keying (OOK) and its reduced peak power, which mitigates nonlinear effects in fiber-optic applications. This has led to demonstrated utility in long haul applications, with experiments confirming more than 3 Tb/s capacity using 80 Wavelength Division Multiplexed (WDM) DPSK channels. It is expected that the first wideband telecommunications fiber optic links using WDM DPSK will be deployed by 2006. DPSK is also an attractive modulation format for high-rate spectrally efficient Free Space Optical (FSO) links because the increase in sensitivity over OOK allows for a corresponding reduction in costly transmitter power.
DPSK conveys information by encoding a relative phase difference between two optical bits. Differential encoding simplifies the receiver by eliminating the need for a stable absolute phase reference that is required for coherent homodyne Phase Shift Keyed (PSK) modulation. Instead of mixing the received signal with a local oscillator phase reference to determine its phase, DPSK mixes the incoming signal with a delayed version of itself. Consequently the DPSK receiver is often referred to as a “self-homodyne” receiver.